My aunt's vision isn't so great any more, and quite often she'll brake for what she thinks is an animal or car, but is just a shadow. Other times, if the sun is low in the sky, she will not notice other vehicles that really are there.

I just make sure that when I'm a passenger in her car I have my hand resting on the emergency brake, so I'm ready to stop the car if I need to. It's much safer that way.